Modem vehicle suspension systems can be quite complex assemblies, adapting as they must from static conditions of the vehicle at rest (and even there with a range of possible loads) through dynamic conditions imposed in travel by road surface, road slope and pitch and turns, external forces like wind gusts, vehicle speed, load shifts, and all possible combinations of these. To adapt to such conditions most vehicle suspensions to date have employed characteristics like camber, caster, and toe which are set to particular values, the suspension locked in some manner to maintain these values, and then those characteristics not intentionally changed. This system of static suspension setup is an understandable attempt to simplify the complex mechanism used to address the dynamic environment encountered in driving.
The key characteristics of suspension systems are camber, toe, and caster. For the following discussion camber is the most important, it refers to the vertical tilt of a wheel either toward or away from the vehicle center. On a vehicle having opposed transversely paired wheels, like an automobile, when a wheel is tilted top inward the wheel is said to have negative camber, and when it points top outward it is said to have positive camber. Similarly, changes in the camber of a wheel may be referred to as being more negative or more positive.
In contrast, toe is the horizontal tilt of paired wheels either together or apart. When the fronts of the wheels tilt inward the wheels are said to have toe-in, and when they point outward they are said to have toe-out. It should be noted that toe deals with a characteristic of wheel orientation which is both horizontal and longitudinal relative to the vehicle. Caster is the forward (negative caster) or rearward (positive caster) tilt of the steering axis of a wheel. For example, in most bicycles the front fork is almost always mounted tilted back, giving the front wheel positive caster. It should be noted that caster deals with a characteristic of wheel orientation which is both vertical and longitudinal relative to the vehicle. Finally, track is the separation between transversely separated wheels. Track is not usually discussed as a suspension setup characteristic, but it is important in the following discussion.
FIG. 1 (background art) depicts a vehicle 12 (in ghost outline) on a horizontal road 14. The vehicle 12 has a conventional suspension system 16 which includes a spacing member 18, which in actuality may be a more complex assembly than is shown. The spacing member 18 has a fixed horizontal displacement between its opposed ends 20. At each end 20 a joint 22 is provided where the upper end of an arm 24 is attached. At the lower end of each arm 24 is a spindle 26, upon which a wheel 28 is rotatably mounted (denoted as a left wheel 28l and a right wheel 28r in the figures; typically the wheels 28 will include tires, but the discussion herein will not generally treat these separately). In FIG. 1 the wheels 28 are shown oriented to true vertical (i.e., zero camber). The vehicle 12 further includes a center of gravity “CG 30,” which for the present discussion will always be assumed to be fixed at the transverse center of the vehicle 12. A vertical center axis 32 is projected through the CG 30 to the road 14, thereby dividing the overall track at the illustrated end of the vehicle 12 into a left track 34l and a right track 34r. The suspension system 16 is depicted simplistically here with components like springs, steering linkages, etc. omitted to facilitate clarity. Thus, FIG. 1 depicts what has been considered proper wheel alignment and suspension setup during much of human history.
FIG. 2 (background art) illustrates the vehicle 12 at rest with the suspension system 16 set up in a conventional modern manner. The tops of the wheels 28 are tilted outward (somewhat exaggerated for illustration), away from the spacing member 18. To emphasize this a pair of vertical side axes 36, a pair of wheel axes 38, and a pair of arcs 40 are provided to depict the angular separation of these. FIG. 2 thus depicts positive camber. Today a slight amount positive camber is considered desirable by many manufacturers (e.g., Ford Motor company in most of its automobiles; but Daimler Benz is a counter example, using slightly negative setup camber in many Mercedes Benz automobiles). Slightly pre-loading camber away from zero in this manner is motivated by the modem use of flexible components like springs and inflated rubber tires, and the goal of maintaining the camber of the suspension system 16 within a useful range during vehicle 12 use, say, a range extending from slightly positive to zero to negative.
Many factors affect wheel 28 orientation when the vehicle 12 is moving, with some obvious examples having already been mentioned, such as passenger and cargo loading. However, less obvious factors must also be considered, such as the natural tendency of non-driving wheels 28 to spread outward at high speed. FIG. 3 (background art) therefore illustrates the vehicle 12 and its suspension system 16 when engaged in typical straight forward motion at highway speed. The slightly positive camber of FIG. 2 has now become slightly negative.
FIG. 4 (background art) illustrates the suspension system 16 as the vehicle 12 makes a hard unbanked turn to the right. A number of changes can be observed: the camber of the left wheel 28l is now positive, the camber of the right wheel 28r is somewhat more negative, and the left track 34l and the right track 34r are no longer equal. There are a number of factors that interact to bring about these changes. A typical lay person might say that the vehicle 12 is “leaning into the turn” and that the wheels 28 (i.e., the tires) are scrubbing the road.
FIG. 5 (background art) is a free body diagram depicting the forces present in the scenario depicted in FIG. 4. At this point it is assumed that the vehicle 12 is not in such an extreme situation that it is skidding or has lifted a wheel 28 off the road 14. The vehicle 12 is reduced here to three rigidly connected points: a left road contact point 42l, a right road contact point 42r, and the previously noted CG 30. The vehicle 12 has weight (W), depicted by a vector vertically extending out of the CG 30. Countering the weight (W) are a left normal force (Nl) and a right normal force (Nr), respectively depicted by vectors vertically extending out of the left road contact point 42l and the right road contact point 42r The vehicle 12 also has a lateral force (L) effectively acting on the CG force (L) are a left friction force (Fl) and a right friction force (Fr), respectively depicted by vectors horizontally extending out of the left road contact point 42l and the right road contact point 42r. Finally, resultant vectors are shown at all three points (30, 42l, and 42r).
Turning now also to FIG. 6a-b, these respectively depict close-ups of the left wheel 28l and the right wheel 28r as they contact the road 14 in the scenario of FIG. 4. In particular, it should be noted that the road 14 surface contact of the left wheel 28l and the right wheel 28r are not the same, a left footprint 44l here is greater than a right footprint 44r. The net result is as depicted in FIG. 5, where the left friction force Fl is shown with a longer vector than the right friction force Fr, because the left friction force Fl is markedly greater than the right friction force Fr (i.e., Fl>>Fr).
Friction has two forms, static and dynamic. Static friction is exhibited as a force resisting motion beginning (in the example, resistance to the vehicle 12 skidding). However, static friction always has an upper limit determined by the particular materials being brought into contact (the road 14 and wheels 28 here), and once that limit is exceeded movement begins (if only one wheel 28 is affected, then it scrubs the road 14; but if all wheels 28 at an end of the vehicle 12 are affected, then that end skids off the road 14). In contrast, dynamic friction is exhibited as a force countering motion once it has begun, and in most cases it is less than static friction. Therefore, to obtain maximum benefit from friction the obvious course is to keep maximum wheel 28 surface in contact with the road 14 by keeping the left friction force Fl add the right friction force Fr as equal as possible (which also helps keep the friction at any particular wheel 28 below the static friction limit). In the argot of automobile racing this can be stated: all of the rubber needs to meet the road. Herein lies one problem with the prior art; conventional suspensions systems today do a poor job maintaining optimal friction distribution between the wheels 28.
In FIG. 5 the left normal force (Nl) is shown as being much greater than the right normal force (Nr). This is correct because the lateral force (L), operating on the CG 30, applies torque to the vehicle 12. This can be confirmed with analysis with classical mechanics, which will also reveal that if the difference between the left normal force (Nl) and the right normal force (Nr) become too great the right wheel 28r will lift off of the ground (assuming that static friction is not overcome first, and the laterally static example here does not turn into a dynamic one; i.e., the vehicle 12 does not simply skid off the road 14 first). In such a situation, if the driver does not quickly enough reduce the lateral force (L), say by decreasing how sharply the vehicle 12 is being turned, the vehicle 12 will roll over. Thus, also in the argot of racing, once the rubber meets the road it needs to stay there. Herein lies another problem with the prior art; conventional suspensions systems today do a poor job maintaining optimal normal force distribution between the wheels 28.
Up to this point static setup of suspension systems has been discussed, because such is still overwhelmingly used today. However, a statically set suspension cannot produce optimum response in all driving situations. That necessarily requires a system which is dynamically able to respond. Many dynamically controlled suspension systems have been attempted, and discussion of the major ones known to the inventor follows.
One set of dynamic efforts has used detection of the steering wheel or else of the steering assembly to control wheel camber. Miichi et al. in UK patent 2,271,968 teach use of hydraulic actuators to adjust toe and camber in response to steering angle. Harara et al. in U.S. Pat. No. 5,481,458 teach a caster angle control apparatus similarly using steering wheel angle. Abe in Japanese patent 62-268773 teaches camber adjustment of rear vehicle wheels based upon front wheel steering angle as detect at the steering wheel. And Oyama et al. (including Abe) in U.S. Pat. No. 4,971,348 teach an enhanced version of Abe '773, but still one for adjustment of rear wheel characteristics only, and one again based upon front wheel steering angle as detected at the steering wheel.
For completeness more than relevance, Sano et al. in U.S. Pat. No. 4,835,714 teach setting toe and camber in relation to vehicle speed. And Gerin in U.S. Pat. No. 3,278,187 teaches a mechanism for changing elevation of the vehicle. FIG. 3 of Gerin '187 is interesting because it shows a vehicle in a hard left turn with the wheels adjusted contrary to the conventional orientations, but this is accomplished with simple height adjustment rather than by changing wheel camber angles.
Other dynamic efforts have used various force sensors and camber adjustment of all of the wheels of the vehicle. Goldberg et al. in U.S. Pat. No. 4,191,274 and in U.S. Pat. No. 4,371,191 (a continuation-in-part of '274) teach a complex system for adjustment of a number of suspension characteristics, including camber, based upon computation of vehicle roll attitude and use of a number of possible sensors, including a centrifugal force sensor. And Serizawa in Japanese patent 62-268770 teaches the use of lateral acceleration detecting means for adjustment of all four vehicle wheels to zero or slightly negative camber.
Of the above, the Goldberg '274 and '191 and the Serizawa '770 efforts are most relevant here. As noted, Goldberg '274 and '191 use very complex systems, employing large numbers of components and concurrently attempting to control camber, caster, toe, vehicle height, and “wheel height” (a misnomer actually referring to vehicle height in relation to a wheel, the dimensions of the wheel itself are not adjusted). In contrast, Serizawa '770 is elegantly simple, but it is limited to setting camber to zero or slightly negative, i.e., it “maintains” camber near static suspension system ideals, rather than employing it fully to counter skids and rolls, and to enhance vehicle turning ability.
In sum, all prior art techniques have used camber adjustment as if the goal were merely to achieve equivalence to static suspension system setup for straight forward driving (i.e., to convert the situation depicted in FIG. 4 (background art) into one of those depicted in FIGS. 1-3 (background art)). This will equalize the left footprint 44r and the right footprint 44r, and the left track 34l and the right track 34r, but because of lateral force (L, in FIG. 5) this will not optimally balance the left normal force (Nl) and a right normal force (Nr) in turns, which is also necessary to achieve optimum cornering capability and safety.